Recently, photo-acoustic tomography has been proposed which determines the distribution of an optical characteristics value in a living organism, with good accuracy, by using the characteristics of an acoustic wave (typically, an ultrasound wave) that produces little scattering inside a living organism compared to light (see Non Patent Literature 1). In the present specification, photo-acoustic tomography is referred to as “PAT”.
When pulse light generated by a light source is irradiated onto a living organism, the pulse light propagates while diffusing in the living organism. An absorbing body contained in the living tissue absorbs energy of the propagated pulse light and generates an acoustic wave. By analyzing this acoustic wave signal, it is possible to obtain an optical characteristics distribution inside the living organism, and in particular, a distribution of the light absorption density.
One reported clinical application of PAT is photo-acoustic mammography (referred to as “PAM” in the present specification). A PAM apparatus detects a position of a tumor in a breast, principally by creating an image of new blood vessels created in the area of a tumor when a tumor is formed, and a region of high absorption coefficient including the new blood vessels. In one example of a reported PAM apparatus, a breast is sandwiched between two parallel flat plates which are located substantially in parallel, an ultrasound wave probe is disposed on one flat plate, and a light beam is irradiated from the other, opposing flat plate (in the present specification, this is known as “front detection type PAT”) (see Non Patent Literature 1).
Moreover, a photoacoustic wave microscope for observing living organisms which uses photoacoustic wave technology has been reported (see Patent Literature 1). In a photoacoustic wave microscope, an ultrasound wave probe which forms a detector and pulse light which is irradiated onto the living organism are disposed on the same side of the living organism (in the present specification, this is referred to as “rear detection type PAT”). The interface region between the ultrasound wave probe and the living organism is not illuminated directly, and the vicinity of the surface of the living organism is depicted in high definition.
In the prior art example of a rear detection type PAT described in Patent Literature 1 mentioned above, it is possible to observe a local position in the vicinity of the surface of the living organism, with high definition. However, since the interface region between the ultrasound wave probe and the living organism is not illuminated directly, then the use efficiency of the light in the deep portions of the living organism is diminished. Therefore, the method described in Patent Literature 1 has a problem in that it is not suitable for objects which require observation in a broad range, up to an including deep portions of the object, such as a breast. In response to this, in recent years, there has been investigation into a method which appends a rear detection mechanism to a PAM apparatus having the composition of the front detection type PAT described in Non Patent Literature 1, and irradiates light from both surfaces of a breast.